1. Field of the Invention
This invention pertains to normalizing and particularly with a normalizer that operates continuously and without requiring resetting of variable control settings.
2. Description of the Prior Art
Sorting of products according to color differences, typically employed for sorting small food products such as rice grains, beans, seed and the like, is commonly performed by automatic sorting machines incorporating optical detectors. Such a machine generally employs a hopper for receiving a bulk of the product to be sorted, a vibratory feeder for determining the flow rate of the product exiting from the hopper, and one or more channels or slides for the product from the feeder. While the product is sliding down a channel, it is aligned and singulated so that as the product leaves the slide it is in a "stream" of product which can easily be "observed" using photodetectors.
The area behind the stream is painted or otherwise treated to have a background appearance for photodetection purposes which is substantially identical to the appearance of an acceptable product. That is, its color and reflectivity characteristics are the same as an acceptable product. Henceforth, this area will be called the "background". When products within an acceptable range of color are subsequently conveyed in front of the background, the photodetector detects nothing out of the ordinary and hence does not cause rejection of any of the product. However, when a product is conveyed which is either too dark in color or which is too light in color, this is a photodetectable non-acceptable product event. A signal is produced which subsequently produces an air jet or the like and the product is ejected from the channel. "Nonacceptable" may not mean that the rejected product is too ripe or not ripe enough or bruised, but merely not of the grade for which the sorter is preset to accept.
It is further true that machines of this type typically have many channels, each channel being subject to independent photodetection sorting in the manner described. This is because a single beam-width is capable of photodetection only with regard a compatible width channel. Furthermore, many of the products that are sorted create a dust environment as particles of the products are knocked free during product conveying. Hence, the optical lenses of the photodetection system are gradually dimmed over a period of time. Furthermore, the machines vibrate which may have an affect on changing adjustable potentiometers and the like. Machines that operate for a long period of time also heat up, which often cause additional changes in the values of electronic components. Finally, over a period of time, the aging of components often causes voltage changes. All of these factors, and perhaps others, have created a need for "normalizing" the sorting machines of the types described above during operation.
One type of normalizing employs two operational amplifiers, the second stage being capacitively coupled to the first. The first-stage amplifier is a high gain amplifier necessary for amplifying the small signals produced by the photodetector. This amplifier is followed by a gain adjustment. The capacitive coupling removes dc offset that is inherent in the first amplifier. The second amplifier receives another input against which the input from the first stage is compared to produce a zero output when the photodetector detects only background color. The gain adjustment provides a necessary variable component for this purpose. In addition, the second amplifier includes a null adjustment potentiometer for compensating for the offset developed in the second stage.
A third stage is dc coupled to the second stage, the third stage including a comparator with respect to an adjustable reference voltage setting. A product that is lighter in color than normal produces a positive voltage signal at the input to the third stage comparator. When this voltage is more positive than the reference voltage setting, the output of the comparator produces an output that results in rejection of the product. A similar comparator circuit is used for rejecting products which have a darker than normal color. It may be seen that it is extremely important that the output of the second stage remain constant at zero volts when no product is in view. Should the light level change as a result of lamp aging, dust accumulation on the viewer window, or the like, the output of the photodetector and thus the output of the first stage changes. For slow and gradual changes in condition, the capacitor charges to compensate. Of course rapid changes are coupled through to the second stage and, if large enough, result in a rejection of a nonacceptable item within the product stream.
Although the circuit described works fairly well, when several consecutive positive signals or several consecutive negative signals are received, there is a shifting of the apparent offset baseline, which will cause the circuit not to operate in rejection of products as intended. Further, the null adjustment of the second stage is a nuisance, particularly when there are many channels for each machine and two or more circuits for each channel: for instance, one for viewing the front of the product and one for viewing the rear of the product. Sometimes there are additional circuits involving "viewing" each separate channel from several perspectives, thereby multiplying the number of null adjustments accordingly.
Another type of normalizing circuit that has been employed that eliminates capacitively coupling together the first and second stages is a rather complicated circuit that ensures that the reference voltage to the second stage is adjusted when the background reference voltage changes outside of an acceptable range. This is reflected in a drifting away from the preestablished output of the first stage. The procedure includes periodically shutting off the machine to ensure only the presence of the background and then providing a correct feedback voltage from a digital-to-analog converted to place the second stage amplifier output at zero volts. In order that the circuit operates in the desired manner, logic circuitry including an oscillator for producing a clock pulse, electronic switches for stopping and staring the machine, sampling and reset networks are required. In short, this procedure is both complicated and expensive. Further, rapid dust accumulation that occurs between the periodic normalizing sequences just described are not compensated for at all.
A third type of normalizer utilizes a separate set of photodetectors for detecting when no products are in view of the sorting photodetectors. An electronic switch is closed so as to permit observing the background and changing the stored charge on the capacitor coupling the first and second stage amplifier with a reference voltage so that such stored charge value is equal to the difference between what the output of the amplifier is and what it should be. Again, switching is required and only then during the non-presence of product in the viewing area. Normalizing may not be performed often enough because of lack of interruptions in the product stream to dependably cause the overall system to function correctly as intended. Further, the compensating charge on the capacitor may decay if not reset over a period of time, resulting in a false normalizing operating change.
Therefore, it is a feature of the present invention to provide an improved normalizing network for a product sorting circuit which does not require an initial nulling adjustment, is unlikely to result in false baseline shifting, does not require shutdown of operation for normalizing and does not require detection of the nonpresence of product in the viewing area in order to provide normalizing.